This application relates to a method and apparatus for controlling the solidification of materials such as metals.
In the art of preparing shaped or cast objects by solidification of molten materials such as metals within a mold, it is a recognized goal to control and improve the mechanical properties of the finished product by controlling the solidification process. For example, methods for obtaining castings having a desirable fine grain size have been proposed using low temperature casting, seeding and mechanical or electromagnetic stirring. Haida et al., Trans. Iron Steel Inst. Japan 24 (1984), p. 891; Mizukami et al., Trans. Iron Inst. Japan. 24 (1984), p. 923.
Vibration during solidification is also thought to cause grain refinement by "breaking off" or "melting" the arms of the developing grains. Haida et al., Trans. Iron Steel Inst. Japan 24 (1984), p. 891; Mizukami et al., Trans. Iron Inst. Japan. 24 (1984), p. 923; Jackson, Trans. Metall. Soc. AIME, 236 (1966), p. 149; O'Hara et al., Trans. Metall. Soc. AIME, 239 (1967), p. 497; Johnston et al., Trans. Metall. Soc. AIME, 233 (1965), p. 1856; Tiller et al., "The Solidification of Metals", Iron Steel Inst., Pub. No. 110 (1967), p. 27; Wu et al., Metall. Trans. A, 19A (1988), p. 1109; Cibula, J. Inst. Metals., 76 (1949) p. 34; Southin, J. Aust. Inst. Met., 10 (1965), p. 115; Kura et al., Battle Memorial Inst. Memorandum, No. 166 (May 15, 1963); Alder, The Aust. Eng., (Dec. 8, 1952), p. 53; Hiedemann, J. Acoust. Soc. Am., 26 (1954), p. 831; Tesman, Met. Prog., 79 (1961), p. 79. Grain refinement is achieved by generating strong shear forces in the solidifying liquid metal in a process known as "Rheocasting." Spencer et al., Metall. Trans., 3 (1972), p. 1925; Mehrabian et al., Trans. Am. Foundrymen's Soc., 80 (1972), p. 173; Fascetta et al., Trans. Am. Foundrymen's soc., 81 (1973), p. 95; Young et al., Trans. Am. Foundrymen's Soc., 84 (1976), p. 169; Matsumiya et al., Metall. Trans. B, 12B (1981), p. 17; Laxmann et al., Metall. Trans. A, 11A (1980), p. 1927; Joly et al., J. Mater. Sci., 11 (1976), p. 1393.
The use of electric current to control solidification has also been disclosed. For example, the passage of electric current through solidifying metals has been shown to have an impact on liquid/solid interface stability. Warner et al., Met. Trans., 4 (1973), p. 1245; Angus et al., Met. Soc. Conf., 8 (Inter Science, N.Y., 1961), p. 833; Pfann et al, Trans. TMS-AIME, 224 (1962), p. 1139; Wagner et al., Trans. TMS-AIME, 236 (1966), p. 554; Verhoeven et al., Trans. TMS-AIME, 239 (1967), p. 694; Hurle et al., J. Mat, Sci., 2 (1967), p. 46. The current densities utilized in these studies were significantly high, i.e., well over one thousand A/cm.sup.2. Solidification of superalloys in an electric field leading to finer precipitation and improved material properties is disclosed by Ahmed et al., Adv. Mater & Proc. 10, 30 (1991). Recently, pulse electric discharging using pulses of high voltage or current applied to the liquid or semisolid metal. Nakada et al., ISIJ Int'l 30, pp. 27-33 (1990). This generates a large electron pressure gradient in the molten metal, affecting the cast structure.
It is an object of the present invention to provide an alterative method and apparatus for solidification of molten materials which relies upon the continuous passage of low electrical current densities through the molten materials at relatively higher potential.